One common step that may be performed frequently during fabrication of modern semiconductor devices is the formation of a thin film on a semiconductor substrate by chemical reaction of gases. Such deposition processes are referred to generally as chemical vapor deposition (“CVD”) and include both thermal CVD processes and plasma-enhanced CVD (“PECVD”) process. In conventional thermal CVD processes, reactive gases are supplied to the substrate surface, where heat-induced chemical reactions take place to form the desired film. In a conventional plasma process, a controlled plasma is formed to decompose and/or energize reactive species to produce the desired film.
Any of these CVD techniques may be used to deposit conductive or insulative films during the fabrication of integrated circuits. One important physical property of CVD insulative films includes the film's ability to fill gaps completely between adjacent structures without leaving voids; this property is referred to as the film's gapfill capability. Gaps that may require filling by CVD insulative layers, such as silicon oxide based layers, include spaces between adjacent raised structures such as transistor gates, conductive lines, etched trenches, stacked capacitors or the like.
As semiconductor device geometries have decreased in size over the years, the ratio of the height of such gaps to their width, the so-called “aspect ratio,” has increased dramatically. Gaps having a combination of high aspect ratio and a small width present a particular challenge for semiconductor manufacturers to fill completely. In short, the challenge usually is to prevent the deposited film from growing in a manner that closes off the gap before it is filled. Failure to fill the gap completely results in the formation of voids in the deposited layer, which may adversely affect device operation. The semiconductor industry has accordingly been searching aggressively for techniques that may improve gapfill capabilities, particularly with high-aspect-ratio small-width gaps.
High-aspect-ratio gaps are difficult to fill using conventional CVD methods, causing some integrated-circuit manufacturers to turn to the use high-density-plasma CVD (“HDP-CVD”) techniques. The use of an HDP-CVD technique results in the formation of a plasma that has a density approximately two orders of magnitude greater than the density of a conventional, capacitively coupled plasma. Examples of HDP-CVD systems include inductively coupled plasma (“ICP”) systems and electron-cyclotron-resonance (“ECR”) systems, among others. There are a number of advantages of plasma-deposition processes in gapfill applications that are thus enhanced in the case of HDP-CVD deposition processes. For example, the high reactivity of the species in any plasma deposition process reduces the energy required for a chemical reaction to take place, thereby allowing the temperature of the process to be reduced compared with conventional thermal CVD processes; the temperatures of HDP-CVD processes may advantageously be even lower than with PECVD processes because the species reactivity is even higher. Similarly, HDP-CVD systems generally operate at lower pressure ranges than low-density plasma systems. The low chamber pressure provides active species having a long mean-free-path and reduced angular distribution. These factors contribute to a significant number of constituents from the plasma reaching even the deepest portions of closely spaced gaps, providing a film with improved gapfill capabilities.
Another factor that allows films deposited by HDP-CVD techniques to have improved gapfill characteristics is the occurrence of sputtering, promoted by the plasma's high density, simultaneous with film deposition. The sputtering component of HDP deposition slows deposition on certain features, such as the corners of raised surfaces, thereby contributing to the increased gapfill ability of HDP deposited films. Some HDP-CVD processes introduce an inert element that further promotes the sputtering effect, with the choice of inert element often depending on its atomic or molecular weight, a parameter that is generally correlated with the size of the sputtering effect. In addition, the sputtering effect may be further promoted by applying an electric bias with an electrode in the substrate support pedestal to use electrical attraction of the plasma species.
It was initially thought that the simultaneous deposition and etching provided by HDP-CVD processes would allow gaps to be filled in almost any application. Semiconductor manufacturers have discovered, however, that there is a practical limit to the aspect ratio of gaps that HDP-CVD deposition techniques are able to fill. The challenge of filling gaps with HDP-CVD is illustrated schematically with the cross-sectional views shown in FIGS. 1A and 1B. FIG. 1A shows a vertical cross section of a substrate 10, such as may be provided with a semiconductor wafer, having adjacent raised features 20, which may be adjacent metal lines, trench walls, or a variety of other structures. Adjacent features 20 define gaps 14 that are to be filled with dielectric material, with the sidewalls 16 of the gaps being defined by the surfaces of the features 20. As the deposition proceeds, dielectric material 18 accumulates on the surfaces of the features 20, as well as on the substrate 10, and forms overhangs 22 at the comers 24 of the features 20. As deposition of the dielectric material 18 continues, the overhangs 22 typically grow faster than the gap 14 in a characteristic breadloafing fashion. Eventually, the overhangs 22 grow together to form the dielectric film 26 shown in FIG. 1B, preventing deposition into an interior void 28.
In one commonly used process, an HDP-CVD process is used to deposit a silicon oxide film using a process gas that includes monosilane SiH4, molecular oxygen O2, and argon Ar. While such a process has been successfully used to fill certain narrow-width, high-aspect-ratio gaps for many applications, improved and/or alternative techniques are desired.